Small patch: initdb: "'" for QUOTE_PATH (non-windows)

Hello Postgres Hackers!

I sent this already a few hours ago but it got blocked because I had not
yet joined the mailing list. Trying again, sorry for any redundancy or
confusion!

This is to fix an issue that came up for me when running initdb.

At the end of a successful initdb it says:

Success. You can now start the database server using:
pg_ctl -D /some/path/to/data -l logfile start

but this command did not work for me because my data directory
contained a space. The src/bin/initdb/initdb.c source code
did already have a QUOTE_PATH constant, but it was the empty
string for non-windows cases.

Therefore, added quotes via existing QUOTE_PATH constant:

pg_ctl -D '/some/path/to/data' -l logfile start

Best,
Ryan